Signaling system



5 Sheets-Sheet l INVE'NroRs Jhn H UDSS Harri.) E' mahunej WMM/,$447, Man/ZL ATTORNEYS Dec. 30, 1-941. H. P. MAHONEY ET Ax.

SIGNALING SYSTEM Filed Deo. 19, 195e Dec. 30, 1941. H. P. MAHONEY ETAL SIGNALING SYSTEM Filed Dec. 19, 193s 5 sheetssheet 2 @SUA Har

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' ATTORNEYS Dec. 3o, 1941. H PQ MAHQN'EY ET AL 2,267,987

S IGNALING SYSTEM Filed D60. 19, 1938. 5 Sheets-Sheet 4 1 07 V v Y 1w k l0 v S5 424 422-1423 70 QQ /ne Circa/fs Finder Lme4l7rcwZL F1. 4 424 P5K. 460 ,213x470 INVENTORS Harz@ E PahnneH JUhnH;Uu55

5 Sheets-Sheet 5 H. P. MAHONEY ETALv Dec. 30, 1941.

' SIGNALING SYSTEM Filed Dec. 19, 1958 Patented Dec. 30, 1941 SIGNALING SYSTEM Harry Paul Mahoney, Oak Park, and John Henry Voss, Downers Grove, Ill., assignors to Associated Electric Laboratories, Inc., Chicago, Ill., a corporation'of Delaware Application ncemberils, 193s, seran No. 246,616

53 Claims.

The present invention relates to signaling systems and more particularly to automatic telephone systems lprovided with main and, branch exchanges.

In a telephone system of the previously mentioned type, apparatus is provided for the purpose of setting up a connection between one line and any other line in the system, which conventionally comprises finder and connector switches having access to each line in the system and a number of intermediate tralc concentrating switches interposed between the finder and connector switches. In a small telephone system, intermediate concentrating switches are unnecessary and the finder and connector switches are directly arranged in links, the finder switch of each link being operative to nd ya calling line in the system and the connector switch of each link being operative to select a desired called line in the system. Each of the connector switches is usually of the numerical trunk hunting type and is so connected and arranged that it is operative in response to a predetermined series Yof digits dialed thereinto to select a corresponding line extending to a subscriber substation and operative in response to a different predetermined series of digits dialed thereinto to select the rst trunk linein a corresponding group of trunk lines extending to a branch exchange. The connector switch vthen operates automatically to select the second trunk line in the group of trunk lines in the event the first trunk line in the group of trunk lines is busy. More particularly, the connector switch operates automatically as a trunk hunting switch to search for an idle trunk line in the group of trunk lines. In the event an idle trunk lineis found, the idle trunk line is selected andv further operaton of the connector switch is arrested.

Frequently, due to increased trafc demands in an existing telephone system, it is necessary to extend additional trunk lines between the main and one or more of the branch exchanges. Although the links of the telephone system are usually able to accommodate additional trunk lines, it frequently occurs that additional trunk lines cannot be accommodated without assigning additional directory numbers thereto, due to the fact that the group of lines including the group of trunk lines in the connector switches of the links are all in use and that the available lines are included in another group of lines in-the connector switches of the links. In view of .the fact that many of the branch lexchanges comprise private branch exchanges or P. B. X, the provision of a second group of trunk lines, extending between the main exchange and the P. B. X and having a directory number different from that of the first group of trunk lines extending between the main exchange and the P. B. X, would be highly objectionable due to the commercial good-will which the P. B. X subscriber has acquired in connection with his directory number. It has been proposed to ob- Viate this objection by providing a number of auxiliary hunting switches individually associated with the connector switches of the links and having access to the second group of trunk lines extending between the main exchange and the P. B. X, each of the hunting switchesbeing operative to search for an idle trunk line inthe vsecond group of trunk lines extending between the main exchange and the P. B. X in the event the associated connector switch ofthe link fails to rind an idle trunk line in the rst group of trunk lines extending between the main exchange and the P. B. X.

While the above-described arrangement is satisfactory in operation, it has not been generally accepted commercially due, primarily, to the large number of auxiliary hunting switches required therein. More particularly, it is noted that, in the above-mentioned arrangement, an auxiliary hunting switch `must be individually associated with the connector switch of eachvlink and be individual to a particular one of the P. B. X having both first and second groupsof trunk lines extending thereto. Thus, for example, even in a small telephone system having only two P. B. X having both rst and second groups of trunk lines extending thereto and comprising only ten links, twenty auxiliary hunting switches are required in the above-mentioned arrangement.

Accordingly, it is an object of the present invention to provide, in a telephone system of the type described, an improved arrangement forv accommodating a second group of trunk lines Yextending to a branchexchange and having the samedirectory number as a lirstgroup of trunk lines extending thereto, which arrangement comprises a minimum amount of apparatus connected and arranged in an extremely simple manner.

Another object of the invention is to provide, in a telephone system of the type described, an improved arrangement for accommodating la number of second groups of trunk lines extending/to a number of different and corresponding branch exchanges and having the same directory numbers as a number of first groups of trunk lines extending to the number of different and corresponding branch exchanges.

It is another object of the invention to provide a signaling system comprising a group of lines and improved switching apparatus for searching for a line having a predetermined condition thereon in the group of lines.

A further object of the invention is to provide an improved signaling system comprising a number of groups of lines and improved switching apparatus for searching in a predetermined order the groups of lines for a line therein having a predetermined condition thereon.

It is a further object of the invention to provide a signaling system comprising a number of lines and improved switching apparatus for setting up connections between the various lines of the system.

' The Afeatures of the present invention are illustrated as being incorporated in an automatic signaling or telephone system including a main exchange, a number of private branch exchanges or P. B. X, main groups of trunk lines extending between the main exchange and the P. B. X, auxiliary groups of trunk lines` extending between the main exchange and some of the P. B. X, and a number of subscriber lines terminating at the main exchange. Also, the system comprises switching apparatus including a number of main links, each comprising a finder switch and a connector switch, a number of overow links, each comprising a finder switch and a hunting switch, a main distributor for assigning the main links for use, and an overflow distributor for assigning the overflow links for use.

In accordance with one feature of the invention, the telephone system is so connected and arranged that the main distributor is operative in response to the creation of a calling condition on any one of the lines in the system to assign an idle one of the main links for use; whereupon, the finder switch of the assigned main link, having access to each line in the system, operates automatically to nd the calling line. The connector switch of the assigned main link is then rendered operative selectively to select any subscriber line and to search for an idle trunk line in any one of the main groups of trunk lines extending `to the P. B. X in the system. Also means is provided in each of the connector switches for causing ringing current to beprojected over an idle subscriber line or trunk line selected thereby and busy tone current to be returned over the calling line in the event the line selected thereby is a busy subscriber line or in the event no idle trunk line is available in the main group of trunk lines searched thereby, provided there is no auxiliary group of trunk lines associated with the main group of trunk lines searched thereby. Also, an arrangement is provided which is operative in the event there is an auxiliary group of trunk lines ,associated with the previously mentioned main group of trunk lines searched by the connector switch of the assigned main link for causing the connector switch, after having failed to find an idle trunk line in the previously mentioned main group of trunk lines, to cause an overflow trunk individually associated with the connector switch to be marked and to cause the overflow distributor to assign an idle one of the overflow links for use. Furthermore, control apparatus is provided for causing the finder switch of the assigned over.-

ow link, having access to the group of overflow trunks individually associated with the connector switches of the main links, t-o operate automatically to search for the marked overflow trunk in the group of overflow trunks and to select the marked overflow trunk.

In accordance with another feature of the invention, the hunting switch of the assigned overfiow link is so connected and .arranged'that it has access to each of the main and auxiliary groups of trunk lines extending to P. B. X having both main and auxiliary groups of trunk lines extending thereto, and is operative automatically to search for an idle trunk line in only the previously mentioned main group of trunk lines and the rprevi-ously mentioned auxiliary group of trunk lines associated therewith, at this time.

` More particularly, the hunting switch of the assigned overflow link is operative to search for an idle trunk line in the previously mentioned auxiliary group of trunk lines and then to search for an idle trunk line in the previously mentioned main group of trunk lines. Also, appanatus is incorporated in the overflow distributor which is effective to cause the hunting switch of the assigned overflow link to be operative for a predetermined time interval or through a predetermined number" of cycles of operation thereof, in order repeatedly to Search for an idle trunk line in the previously mentioned main and auxiliary groups of trunk lines, and then to arrest operation thereof. The previously mentioned means included in the connector switch of the assigned main link is operative to cause ringing current to be projected over a trunk line selected by the hunting switch in either the previously mentioned main or auxiliary groups of trunk lines, and operative to cause busy tone current to be returned over the calling line in the event the hunting switch of the assigned overflow link fails to nd an idle trunk line in either the previously mentioned main or auxiliary groups of trunk lines during the abovementioned predetermined operation thereof. Also, an arrangement is included in the connector switch of the assigned'main link for connecting the calling line to the trunk line selected by the hunting switch of the assigned overflow link after a call is answered at the P. B. X to which the selected trunk line in the previously mentioned main or auxiliary groups of trunk lines extend, the call being routed between the calling line and the selected P. B. X trunk line by way of the assigned main link, the overflow trunk individually associated with the connector switch of the assigned main link and the. assigned overflow link.

Further features of the invention pertain to the particular arrangement of the circuit elements of the system and to the particular connection and arrangement of the elements of the switching apparatus incorporated therein, whereby the above-outlined and additional features are attained.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to thev following specification taken in connection with the accompanying drawings, in which Figures 1 to 5, inclusive, taken together, illustrate diagrammatically the details of a telephone systenlrincluding `an automatic central exchange and a number ci. private branch exchanges and having incorporated therein the present invention.

Description of-the apparatus incorporated in the telephone system Referring now more particularly to Figs 1 to 5, inclusive, of the drawings, 'the telephone system there illustrated comprises an automatic central exchange, a number or" private branch exchanges 'or P. B. X, including the P. B. X indicated at 456, 45B and 410, and a number of P-.B. X trunk lines extending between the central exchange and each of the P. B. X and terminating at the central change.` The P. B. X trunk lines extending from the central exchange to the various P. B. X are arranged in groups, at least some of the P. B. X having main and auxiliary groups o'f P. B. X trunk lines extending thereto. For example, each of the P. B. X 450, 465 and 470 have a main group of P. B. X trunk line'sextending thereto, while the P. B. X460 and Mil also have an auxiliary group of P. B. X trunk lines extending thereto. More particularly, the main groups of P. B. X trunk lines 42B, 425 and 430, comprising the P. B. X trunk lines 112i, 422, 423 and 426, 42T, 428 and 43|, 432, 433, respectively, extend to the P. B. X 450, 410 and 45S, respectively; and the auxiliary groups of P. B. X trunk lines '440 and 445, comprising the P. B. X trunk lines 441, 442, 443 and 446, 44?', 44S, respectively, extend to the P. B. X 460 and Alli, respectively. Also, a number of subscriber lines, including the subscriber lines -I and 44u, are terminated at the central exchange. Furthermore, the central exchange comprises a number of 'main links, including the illustrated lifnli it, and a number of overflow links, including the illustrated link Si), which are employed in setting up Ythe connections between the various lines. in a `manner more fully described subsequently. More specifically, the central exchange has an ultimate capacity of one hundred lines. The number kol" main links included in the system isV determined by the total amount of traino which thei central exchange is required to handle and usually 'satisfactory service may be rendered when the central exchange is provided with ten main links. lThe number of overflow links included in the system is determined by the amount of P. B. X trame which the central exchange is required to handle and usually the number of overflow links provided in the central exchange corresponds to the number of P. B. X having auxiliary groups of trunk lines extending thereto. n'thecentral exchange, a distributor 50 is provided for the purpose of assigning the main links for use in a predetermined order; and a distributor lll is provided for the purpose of assigning the overflow links for use in a predetermined order.

Each of the main links may be identical to the link 12B which comprises a nder portion or switch 3l] and -a connector portion or switch 40, the dnt-ier switch 38 having direct access to each subscriber line, to each main P. B. X trunk line and to each auxiliary P. B. X trunk line in the syst-ern, and the connector switch 40 having direct access to leach subscriber line and to each main P. B. X trunk line in the system. Also, thef'cnnector switch 4B has direct access to the overiiow t-runkill individually associated therewith. Each of the overow links may be identicaltofthe '-link 68 which comprises a -nder por- 62', the-'finder switch ilhaving direct access to eachover-flow trunk in the system, and the hunt` ing' switch 62 having direct access to each auxilifary P. B. X- trunk line and to each main P. B. Xy

tru-nk line extending to. all?. B. X having a group of auxiliaryP. B. -X lines extending thereto in thelsy'st'em. Accordingly, the connector switch ofgeaohrnain link has access' to each auxiliary P. 5B. X trunkli'ne only by way of one of the overflowlinks.

A subscriber substation is associated with each ofthe subscriber lines, the subscriber substatiens indicated at A .and B being associated with the subscriber-lines H08 Yand 4I0, respectively. Each of the subscriber substations is provided with a telephoneA instrument, a dial mechanism and a ringer. Also, each of the P. B. X is provided with the usual apparatus for extending the calls to the subscriber substations therein and usually comprises .an operators `position provided with a telephone instrument, a dial mechanism and a signal device thereat. Also, a line circuit is individually associated with'l each of the subscriber lines Aandeach of the P. B. X trunk lines, the'line circuits individually associated with the subscriber lines |00 and 4H) being indicated respectively at lill Aand 4H, the line circuits individually associated Awith the P. B. X trunk lines 42l, 422, 423 and 426, 421, 428-, 445, 441, 448 and 43l, 432, 433, 4M, .442, 443 being indicated at 424, 428- and 434, respectively. Preferably, the line 'circuits are of identicalconnection and arrangement and may be of any of the well known types.` o Y Preferably, each of the main links is connected and arranged in a manner similar to that of the link 2i). which comprises, Yas best shown in Figs. l and 2,- the nder switch 39 and the connector switch 4t, each of these switches including a mechanism of the Strcwger'y type. The Strowg'er mechanism included in the finder switch 30 is connected and arranged in a conventional manner and hasnot been illustrated in the interest of brevity. The Strowger mechanism included in the connector switch 40, as best shown in Fig. 2, comprises a bank of fixed contacts arranged Yin ten vertical levels, including the illustrated flevel 291), each level including ten ra' set, including the wipers 29| to 295, inclusive,

associated lwith the contact bank and carried by a carriage rmovable, in both vertical and radial directions. The illustrated level 29D in the contact bank may be any level therein, although it is 'preferable that this level be the tenth level. More particularly, it will be observed that the subscriber line 4I0 terminates at the rst contact set in the' level 290; the main group of P. B. X trunk Ylines 42E) terminate at the second, third and '-fourth contact sets in the level 29B; the main group ofP. B. X'trunk lines 425 terminate at the fth, sixth and seventh 'contact sets in the level 290; andthe main group of P. B. X trunk lines 43] terminate `at the eighth, ninth and tenth contact sets in the level 290. Accordingly, it will be understood that the directory number of the subscriber line 450 extending to the subscriber substation B is Ol; and 'the directory numbers of the main groups of P. B. X trunk lines lai/2li, 425 .and 436 extending to the P. B. X 459, 47B and 466, respectively, are 02, 05 and 08, respectively.A Also, this Strowger mechanism includesa vertical magnet M2M for moving the non csr-,switch 6 land a nuntingportion or switchcarriage in the vertical direction, a rotary magnet M206 for moving the carriage vin the radial direction, anda release magnet M|50 for releasing the carriage and for causing the carriage to be returned to its normal-position. Also associated ywith this Strowger mechanism are two sets of switch springs S|5| and S226, the switch springs SI5| being actuated into engagement and the switch springs S226 being actuated into disengagement when the carriage is moved in a vertical direction away from its normal position.

The connector switch 40 also comprises a group of relays and their control and controlled circuits, this group of relays including a line relay RI30, a hold relay RMU, a battery reversing relay R|20, a transfer relay R200, a ring out-off relay R2 |0, a busy relay R230,` a switch-through relay R260, a switch-over relay R210, a trunks busy relay R280, and a number of control relays R220, R240 and R250.

Preferably, each of the overilow links is connected and arranged in a manner similar to that of the link 60 which comprises, as best shown in Fig. 3, the nder switch 6| and the hunting switch 62, each of these switches being of the rotary type and including a number of contact banks and associated wipers. More particularly, the finder switch 6| comprises the contact banks 32|, 325, 33|, 335, l34| and 345 provided with the associated wipers 322, 326, 332, 336, 342 and 346, respectively,'each of these wipers being of the double-endedtype. Also, the finder switch 6| includes a rotary magnet M6| for rotating simultaneously the wipers noted. More particularly, the hunting switch 62 comprises the contact banks 355, 36| and 365 provided with the associated wipers 352, 356, 362 and 366respectively, each of these Wipers being of the double-ended type. Also, the hunting switch 62 includes a rotary magnet M62 for rotating simultaneously the wipers noted. Also, it is noted that the conductors of the overflow trunks extending irom the connector switches of the main links in the system are terminated at the respective and corresponding contacts in the contact banks noted of the finder switch 6I. For example, the conductors of the overflow trunk 300 extending from the connector switch 40 of the illustrated main link 20 are terminated at the respective and corresponding contacts illustrated in the contact banks noted of the finder switch 6|. Furthermore, it is noted that the conductors comprising the main and auxiliary groups of'P. B. X trunk lines extending to the P. B. X 460-and 410 are terminated at the respective and corresponding contacts in some of. the contact banks noted of the hunting switch 62. More particularly, in the arrangement illustrated the rst, second and third respective contacts in the contact banks; 35|, 355 and 36| of the hunting switch 62 terminates the auxiliary P. B. X trunk lines 44|, 442 and 443, respectively, extending to the P. B. X 460; the fourth, fifth and sixth respective contacts in the above-mentioned contact banks of the hunting switch 62 terminate the main P. B. X trunk lines 43|, 432 and 433, respectively, extending to the P. B. X 460; the seventh, eighth and ninth respective contacts in the above-mentioned contact banks of the hunting switch 62 terminate the auxiliary P. B. X trunk lines 446, 441 and 448, respectively, extending to the P. B. X 410; and the tenth,

eleventh and twelfth respective contacts in the above-mentioned contact banks of the hunting switch 62 terminate the main P. B. X trunk lines 426, 421 and 428, respectively, extending to the P. B. X 410. Accordingly, during the operation of the hunting switch 62 the wipers noted respectively and successively engage the contacts in the associated contact banks noted terminating the individual P. B. X trunk lines in the auxiliary group of trunk lines 440, then the individual P. B. X trunk lines in the main group of P.'B. X trunk lines 430, then the individual P; B. X trunk lines in the auxiliary group of P. B. X trunk lines 445, and, finally, the individual P. B. X trunk lines in the main group of P. B. X trunk lines 425.

Furthermore, the link 60 comprises a switchthrough relay R310 .and a transfer relay R380 and 4the 'control and controlled circuits therefor.

The distributor 50, which is operative to assign idle ones of the main links for use, is of entirely conventional connection and arrangement and has not been shown in detail in the interest of brevity.4 However, it is preferable that the distributor 50 is of the preselective type, such that an idle main link is selected to be assigned to the use of a calling line prior to the initiation of a call on a line in the system, and another idle main link is automatically selected to be assigned to the use ofthe next calling line as soon as the selected main link operates to find a calling line in the system.

The distributor 10, which is operative to assign idle ones of the overflow links in a predetermined order, comprises a rotary switch 500 provided with a number of contact banks each having an associated wiper. More particularly, the rotary switch 500 includes the contact banks 50|, 505, 5| I, 5|5, 52| and 525 provided with the wipers 502, 506, 5|2,` 5|6, 522 and 526, respectively,4 each `of these wipers being of the doubleended type. Furthermore, the rotary switch 500 includes a rotary magnet M10 for rotating simultaneously the wipers noted. Also, the distributor 10 comprises a group of relays and their control and controlled circuits, this group of relays including a start relay R530, a trunks busy relay R540, a timing relay R555, a stepping relay R515, a switch-through control relay R510, a transfer control relay R580 and two control relays R550 and R560. It is noted that the control conductors extending to the overow links areterminated at the respective and corresponding contacts in the contact banks noted.

LThe distributor 10 is of the preselective type in that an idle overflow link is selected to be assigned to the use of a calling one of the overflow trunks prior to the creation of the calling condition on the overow trunk, and another overflow link is automatically selected to be assigned to the use of the next calling overflow trunk as soon as the selected overflow link is operated to nd a calling overflow trunk.

Ordinary substation to substation calls n An ordinary call which is to be extended from one subscriber substation to another is initiated at the `calling substation by removing the receiver of the telephone instrument thereatfrom its hook. For example, in extending a call from the subscriber substation A to the subscriber substation B, the call is rst initiated at the substation Alby removing the receiver of the telephone instrument thereat from its hook, thereby to complete a circuit by way of the subscriber line |00 for causing the line circuit |'0| individually associated therewith to be operated in a Well known manner. This operation of the line circuit Illl is eietiveto apply smiled potential to the. startfconductorGllll extending intothe distributorl 59,.r thereby to initiate operation of the di,Str-ibutor.l The distributor 50 then operates to assignrtheV previously selected idle main link to the use oi the `calling subscriber line I00. Assuming that the illustrated main link 20 is assigned to the, use of the calling line |00, the distributorlil initiates operation of the Strowger mechanism included in the finder switch 30. Also, the linecircuit |01 individually associated with-the subscriber line marks both the level and radialposition of the contact set in the contact bank of the Strowger mechanismincluded in the finder switch 30 wherein the calling subscriber line |00 terminates. 'I'he Strowger mechanism included in the nder switch 30 then operates to movethe carriage in the vertical direction tothe level wherein the calling subscriber line |00 terminates, whereupon further movement of the carriage in the vertical direction is arrested. The vStrowger mechanism included in the finder switch 30 then operates to move the carriage in the radial direction to the radial position wherein the calling subscriber line |00 terminates, whereupon further movement of the carriage is arrested. At this time the `calling subscriber line |00 is seized by the nderswitch 32. The iinder switch 30 then operates to connect the calling subscriber line |00 to, the trunklline III), thereby to cause the line and hold relays R130 and Rl, respectively, in the connector switch 40 of theselected link 20 to beoperated in a manner more fully described subsequently. Upon operating, the Vhold relay Rll causes ground potential to be applied to the control `conductor CII2 of the trunk line IIII. This application of ground` potential-to the control` conductorjCi I.2, of .the `trunk line I I 0 causes the-line circuit; I0! individually associated with thecalling subsoriber'line |50 to be further operated, thereby tomark the calling subscriber line 15,0 Aas busy to the connector switches of the links `having access thereto by the application of ground: potential toY the control conductor Clthereof. Further, the line circuit IOI operates to remove the application of ground potential to the start conductor CIlIY extending into the distributor 5D, thereby to cause the distributor 50 to operate and select another idle main link to be assigned to the `use of the next calling line.

As previously noted, when the finder switch seizes the calling line |00 theline relay RI30 in the connector switch 40 is operated, this relay being energized by way of a loop circuit extending between the subscriber substation A and the line relay Ritt; This loop circuit extends from groundat the lower winding of RI30, by way of the lower winding of RISli, lEtCIZl' and RAI25 to the line conductor Ci I3 of the trunk line IIO; and from battery at the upper winding of RISE), by vway of the upper Winding of RI30RCI22 and RAIZto the line conductor CII I of the trunk line H63, the line conductors CH3 and CIII of the trunk line I ibeing connected-'by way of the operated finder switch 30 and the operated line circuit 'mi to the line conductors .CI05 and Cl03 of the calling subscriber line |00, and the line conductors ,02'35 and CHL-'5 of the subscriber line ISS being included ina bridge circuit at the substation A. Upon operating, the linevrelay RI30 completes, at RAll and RCIV32, an obvious circuit for energizing the hold relay RI40. When thus energized,vthel1old relay RI40 operates to isre Reli? 3914.3., e point in LV2l-Circuit, 'traced hereinafter,ior energizing in multiple the veragnetMoil--and the lower winding of the y Ato thel control conductor Ciiigior. ahpurpqse morefully explained subseqlle'flty' .Afl-l ih @fpplaon O ground poten ti L t oleonductor. CH2 completes a nergizingtheiupper winding of the' lay Ric'this circuit extending from d control conductor CI I2 by way of i 1ings S225 and the upper winding battery. `Whenenergized over the `ve.-trex'iegl-l circuil the relay` REM operates to pare,Y atv-RAgiiiland RC20I`, a further point usly mentioned circuit for energizing of the transfer relay R200, and

iitrupts tiene andado, a. point iria ciftracedghereinafter, for energizingy in series the cQntrol` relay R240 and the rotary vmagnet M205. At this time the connector switch 40 is conditioned to be responsive to series of digits dialed. at the substation A in order toA cause the Strowger mechanism included therein tok operate and seize the desiredcalled line.

The subscriber at the substation A then dials the'rst digit ofy therdireotory number ofthe sailed subscriber line at the dial' mechanism thereatQthereby to transmit a first series of irnpulses by way of kthe subscriber loop circuitv to theline relay RI30. The relay RI30 follows the dialed impulses in the usual manner, restoring each time the loop circuit is interrupted, and

operating each time rthe loopcircuit is complet-l ed, `Each time the relay RI30 restores, it internet M203 and the lower winding of the transfer relay R200. The last-mentioned circuit extends from ground at RAI3I, by way Yof RCI33, RAI42, RCM@ and RAEill to RC20I, one branch of this circuit extending from RCZEBI by way of the winding of M208 to battery, and the other branch of this circuit extending from RC20I by way of RCZ'IS", RA2I4 and the lower winding of R200 to battery. The relays RMU and R200 being o f the slow-to-release type'remain operated during -impulsing` Each time thevertical magnet M2513?, isenergized over the above-traced circuit, the carriage of the Strowger mechanism included in the connector switch 40 is moved one step in the vertical direction. When the carriage of this Strowger mechanism is moved away from its normal position one step in ythe vertical direction the-switch springs SISI and S226 are actuated into engagement and disengagement, respectively. When the switch springs SI5I are actuated into engagement, a further point in the, previously mentioned circuit for energizing the release magnet Mlil is prepared, and when theswitch springs S225 are actuated into disengagement; the circuit for energizing the upper winding of the relay R200 is interrupted. At the conclusion of the first digit dialed,l the relay ng,the holdv relay RIM) prepares, at.'

interrupts, at RAI42 and an obvious path for applylethe vertical magnet M208 and the RI30 reoperates, thereby to interrupt, at RAI3| andRCI33, the previously traced circuit for energizingin multiple the lower winding of the relayvR200 and the winding oi the vertical magnet M208. When this circuit it interrupted rfurther operation of the vertical magnet M208 is arrested, thereby to arrest the carriage of the Strowger mechanism included in the connector switch 40 in a vertical position corresponding to the rst digit dialed and to the level in the contact bank of this Strowger mechanism in which the subscriber line extending to the called subscriber substation B is terminated. In the present-instance the called subscriber line 4I0 is terminated in the tenth level 200 in the contact bank of the Strowger mechanism included in the connector switch 40, Accordingly, the rst-digit dialed is the digit 0. Also, when this circuit is interrupted, the circuit for energizing the lower vwinding of the transfer relay R200 is interrupted, thereby to cause the latter relay to restore shortly thereafter. Upon restoring, the relayV R200 interrupts, at RA200 and RCI, a further point in the previously traced circuit for energizing in multiple the verticalmagnet M208 and the lower winding of the relay R200, and prepares, at RA200 and RC202, a further point inthe previously mentioned circuit for energizing in series the control relay R240 and the r0- tary magnet M206.

The subscriber `at the substation A then dials the second digit of the directory number of the called subscriber line at the dial mechanism thereat, thereby -to transmit a second series of impulses by way of the subscriber loop circuit to the line relay RI30. The line relay RI follows they dialed impulses as previously explained. Each time the relay RI30 operates it completes, at'RAISIand RCI32, the previously traced circuit for energizing the hold relay Rl40. Each time the line relay RI30 restores it completes, at RAIBI andRtCl33, the previously mentioned circuit vfor. energizing in series the control relay R240 and the rotary magnet M206. The lastmentionedcircuit extends from ground at RAI 3 l by way of RCI33, RAI42, RCl43, RA200, RC202, RA234, the winding of R240, RA266 and the winding of M206 to battery. The relays RI40 and R240, being of the slow-to-release type, remain operated during impulsing. When-energized over the above-traced circuit, the relay R240 operates to complete, at RA240, a multiple path around the contacts controlled by the armature `RA234 of the busy relay R230 in the pre viously mentioned circuit for energizing in series the re1ay`R240 and the rotary magnet M206. Also, the relay R240 completes, at RA24I and RC242, lan obvious circuit for energizing the control relay R250. v When thus energized, the relay R250 operates to interrupt, at RA250, a point in a circuit, traced hereinafter, for energizing the upper winding of the switch-over relay R210; to interrupt, at RA25I and RC252, a point in a circuit, traced hereinafter, for energizing the upper winding of the switch-through relay R260; and

to prepare, at RA25| and RA253, la point in a circuit, traced hereinafter, for energizing the busy relay R230. Each time the rotary magnet M206 is energized the carriage of the Strowger mechanism 'included in the connector switch 40 is moved one step in the radial direction. At theV conclusion of the seconddigit dialed, the carriage of the Strowger mechanism included in the connector'switch 40 occupies a radial position corresponding to the second digit dialed and causes i the wipers'.l carried thereby to engage the contacts in-the contact `bank of this Strowger mechanism terminating the called subscriber line. In the present instance the called subscriber line 410 is terminated at the first contact setin the level 290 in the contact bank of the Strowgerv mechanism included in the connector switch 40. t dialed comprises the fdigit 1 and the wipers 29|, 296 and 295, respectively, engage the lcontacts in the level 200 terminating the conductors C4ll, CM2 and C4|3 of the called subscriber line 4I0. Also, at the conclusion of the second digit dialed, theline relay RI30 reoperates to interrupt, at RA|3I `and RCl33, a point in the previously traced circuit for energizing in series the control relay R240 and the rotary magnet M2 06, thereby to arrest further operation of the rotary magnet M206 land consequently further movement in a radial direction of the carriage of the Strowger mechanism included in the connector switch 40 and to cause the control relay R240 to restore shortly thereafter.

At this time the called subscriber line 4|0 has been seized by the connector switch 40 and the subsequent operation connector switch depends upon the idle or busy condition of this line. It is noted that ground potential appears upon the control conductor C412 of the called subscriber line 410 in the eventthis line is busy, whereas battery potential appears upon this control conductor in the event this line is idle. Assuming that the called subscriber line 4I0 is busy, the presence of ground potential upon the control conductor C4l2 thereof completes a circuit for energizing the busy relay R230, this circuit extending from the grounded control conductor C4I2 by way of the wiper 293 and associated contact in the level 290, C241, RC215, RA214, RA25I, RAZ 53, RA2 64, RA2 11 and the winding of R230 to battery. When energized over the above-traced circuit, the relay R230 operates to interrupt, at RA23I and RC232, a further point in the previously mentioned circuit for energizing the winding of the switch-through relay R260; to prepare, at RA23I and RC233, a holding circuit, traced hereinafter, for energizing the winding thereof; to interrupt, at RA234, a point in the original circuit for energizing the control relay R240 in series with the rotary magnet M206; and to` prepare, at RA236, a circuit, traced hereinafter, for returning busy tone current over` the loop circuit extending to the calling substation A.' Shortly after the busy relay R230 operates, the control relay R240, being of the slow-torelease type, restores to interrupt, at RA240, a further point in the' circuit for energizing in series the relay R240 and the rotary magnet M206, and to interrupt, at RA24I and RC 242, the circuit for energizing the control relay R250, thereby to cause the latter relay to restore shortly thereafter, the latter relay being of the slow-torelease type. Upon restoring, the control relay R250 completes, at RA253 and RC254, the previously mentioned holding circuit for energizing the busy relay R230, this circuit extending from i the grounded control conductor CH2 by way of RA23I, RC233, RC254, RA253, RA264, RA2'I1, and the winding of R230 to battery. Als'o,^the relay R250 interrupts, at RA25| and RA253, a point in the original circuit for energizing the busy relay R230 and completes, at RA256, the previously mentioned circuit for returning busy tone current over the loop circuit extending to the calling substation A. This loop Accordingly, the second digit circuit extends from the ungrounded terminal` of a grounded busy tone transformer winding,r by way of the busy tone conductor C223, the condenser 229, RA256, RA236, CI35 and the condenser IS'I to the line conductor CII3 of the trunk line IIO; and from batteryI at the upper winding of RI30, by way of the upper winding of RI30, RCI22 and RAI23 to the line conductor CI Il of the trunk line III), the line conductors CI I3 and CIII of the trunk line H0 being connected to the line conductors CI05 and CI03 of the subscriber line I0ll by way of the operated nder switch 30 and the line circuit IUI, as previously noted, When busy tone current is re.- turned over the loop cir-cuit to the calling substation A, the receiver of the telephone instrument thereat is actuated to produce busy tone. The production of busy tone in the receiver of the telephone instrument at the substation A indicates to the subscriber thereat that the `desired connection cannot immediately be had due to the busy condition of the called subscriber line ISIS. The subscriber at the calling substation A then releases the operated main link 20, including the finder switch 30 and the connector switch 40, and the line circuitIIlI by replacing the receiver of the telephone instrument at the substation A upon its hook. The exact manner in which these operated pieces of apparatus are released is described in detail hereinafter.

Assuming that the called subscriber line 4I0 is idle when it is seized by the connector switch 40, battery potential appearing upon the control conductor C4I2 thereof completes a path for shortcircuiting the winding of the busy relay R230, this path being ,substantially the Same as the previously traced path for energizing the winding of R230, thereby to retain the busy relay R230 in its restored position. Also, when the connector switch 40 seizes the called subscriber line 4I0, the line circuit 4I? individually associated therewith operates to mark the subscriber line 4I@ as busy to the connector switches of the other links having access thereto. Shortly after the connector switch 40 operates to seize the called subscriber line M0, the control relay R240 restores to interrupt, at RA24! and RC242, the previously mentioned circuit for energizing the control relay R250, thereby to cause the latter relay to restore shortly thereafter. Upon restoring, the relay R250 completes, at RA25I and RC252, the previously mentioned circuit for energizing the upper winding of the `switch-through relay R260, this circuit extending from battery potential appearing upon the control conductor C4I2 of the called subscriber line 4I0 by way of the wiper 293 and associated contact in the level Zeil, C241, RC2'I5, RA2'I4, RA25I, RC252, the upper winding of R260, RC232 and RA23I to the grounded control conductor CI I2. When energized over the above-traced circuit, the relay R260 partially operates to close the X contacts associated with the armature RA265, thereby to complete a holding circuit for energizing the lower winding thereof, this circuit extending from the grounded control conductor CI'IZL' by way of RA255 and the lower winding of R250 to battery. When the lower winding of the relay R260 is energized over the above-traced holding circuit, this relay fully operates to complete, at RA260, a circuit for returning ring-back tone current over the loop circuit extending to the. calling subscriber substation A, this circuit extending from the ungrounded terminal of a grounded' ring-back tone current transformer winding, by way of the ringfbacktone conductor C226, RA2B0, the condenser 2I9, RC2I4, RA2I3, CI34 and the condenser |36 to the line conductor CI II of the trunk line I I0, and from ground at the lower winding of RI30, by way of the lower winding of RI30, RCI26 and RAI25 to the yline `conductor CI I3 ofthe trunk line H0, the line conductors C`I II and CI I3 of the trunk line IIO being included in the rloop circuit extending by way of the'subscriber line |00 to the calling subscriber substation A, as previously noted. The ring-back tone current returned over the loop circuitl extending tothe calling substation A actuates thegreceiver of the telephone instrument thereat,.thereby to cause the receiver to produce ring-back tone. 'I"ne ring-back tone produced by the receiver of the telephone instrument at the calling substation A indicates to the subscriber thereat lthat ringing current is being projected over the called subscriber line. Also, upon opera-ting, the relay R260 completes, at RAZGI, RC263v and at RAZG'I, a circuit for projecting ringing current over the subscriber line 4I0 extending: to the calledsubstation B, this circuit extending from the ungrounded terminal of a grounded ringing current transformer winding by way of the ringing conductor C221, the upper Winding of the ring cut-off relay RZIIl, RC2I2, RAZII, RC263, RAZSI, RA2II, RC2I2, C245 and the wiper 29I and associated contact to the line conductor C4II of the subscriber line 4I0; and from ground, at RC2 I 8, by way of RA2I0, RA261, RA2'II,r RC2'I3, C249 and the wiper 295 and associated Contact to the line conductor C4I3 of the subscriber'line, the line conductors C'4II and: C4 I3 of the subscriber line being connected together by way of the ringer at the called substation B. The ringing current projected by way of the subscriber line 4'I0 to the called substation B causes the ringer thereat to be actuated, thereby to indicate to the subscriber thereat that the substation B is a called substation.

Thev callis answered at the called substation B by removing the receiver'of the telephone instrument thereat from its hook, thereby to complete a bridge circuit including the telephone instrument thereat between the line conductors CIiI I and CMB of the subscriber line 4I0. This bridge circuit across-the line conductors of the subscriber line completes a low resistance circuit for energizing the upper winding of the ring cutoif relay REID. upper winding of the relay R2I0 is substantially identical to the previously traced circuit for proje'cting ringing current over the subscriber line 4 il. tially operates to close the X contacts associated with the armature RAZ! 5 in order'to com--y plete a holding circuit for energizing the lower Winding thereof, this circuit'extending from the previously traced circuit for returning ring-back tone current over the loop circuit extending to the calling substation A; and to complete, at RAZI i, .RC2I3 and at RA2I0,RC`2I`I, a loop cir cuit extending between the battery reversing refla'y RI20 andthe cal-led substation B. This loop circuit extends from ground at thelower Winde The circuit for energizing the f When `thus energized, the relay R2I0 paring of R120 by way of the lower winding of R120, C135, RC2II, RA210, RA20'1, RA2'II, RC2'I3, C240 and the wiper 295 and associated contact in the level 290 to the line conductor C413 of the subscriber line 410; and from battery at the upper winding of R120, by way of the upper winding of R120, C134, RA2I3, RAZII, RC263, RA261, RAZ'II, RC2`I2, C245, and the wiper 291 and associated contact in the level 290 to the line conductor C411 of the subscriber line 410, the line conductors C413 and C411 of the subscriber line 410 being connected together by way of a bridge path including the telephone instrument at the called substation B. When energized over the above-traced` loop circuit, the battery reversing relay R120 `operates to complete, at RA121, RAI23 and at RA124, RA125, points in an alternative loop circuit extending by way of the subscriber line 100 to the calling substation A; and to interrupt, at RA123, RC122 and at RAI25, RC126, points in the original loop circuit extending by way of the subscriber line 100 to the calling substation A, thereby to reverse the applicati-on of battery potential to the line conductors C111 and C113 of the trunk line .1 I included in the loop circuit extending by way of the subscriber line 100 to the calling substation A. At this time ground at the lower winding of AR130 is connected by way of the lower winding of R130, RAIZI and RAI23 to the line conductor C111 of the trunk line 110, and battery at the upper winding of R130 is connected by way of the upper winding of R130, RAI24 and RA125 to the line conductor C113 of the trunk line I 10. This reversal of battery potential as applied to the line conductors C111 and C113 of the trunk line 110 may be utilized for supervisory purposes in order to operate a call register or the like associated with the trunk line 110. Furthermore, the relay R120 interrupts, at RAI2'I, a further point in the previously mentioned circuit for energizing the release magnet M150 and completes, at RA128 and RC120, a multiple path for applying ground potential to the control conductor C112.

At this time talking battery is supplied by way of the line relay R130 to the loop circuit extending by way of the subscriber line 100 to the calling substation A; and talking battery is supplied by way of the battery reversing relay R120 to the loop circuit extending by way of the subscriber line 410 to the called substation B. Furthermore, the loop circuit extending to the calling substation A is connected by way of the voice frequency current condensers 136 and 131 to the loop circuit extending to the called substation B, thereby to interconnect the calling and called subscriber substations A and B for two-way communication therebetween. More particularly, the line conductors C111 and C113 of the trunk line |10 included in the loop circuit extending to the calling substation A are respectively connected by way of the condensers 130 and 131, respectively, to the line conductors C134 and C135, respectively, the last-mentioned line conductors being included in the loop circuit extending tothe called substation B.

The release of the established connection between the calling subscriber substation A and the called subscriber substation B is primarily under the control of the subscriber at the calling substation A. When the receiver of the telephone instrument at the called substation B is placed upon its associated hook, the loop circuit extend- 'ing to the called substation B is interrupted,

thereby to cause the battery reversing relay R to restore. Upon restoring, the battery reversing relay R120 interrupts, at RAIZI, RA123 and at RAI24, RAI25, points in the second-mentioned loop circuit extending to the calling substation A and completes, at RA123, RCI22 and at RAI25, RC120, points in the original loop circuit extending to the calling substation A, thereby to again reverse battery over the line conductors C1 1I and C113 of the trunk line 110 included in the loop circuit extending to the calling substation A for supervisory purposes. Also, upon restoring, the battery reversing relay R120 prepares, at RA121., a point in the previously mentioned circuit for energizing the release magnet M150; interrupts, at RA128 and RCI29, the multiple path for applying ground potential to the control conductor CI 12'; and prepares, at RAIZ@ and RC120, a point in a circuit, traced hereinafter, for operating a supervisory signal associated with connector .switch 40. When the receiver of the telephone instrument at the calling substation A is replaced upon its hook, the loop circuit extending to the callingy substation A is interrupted, thereby to cause the line relay R130 to restore. Upon restoring, the relay R130 interrupts, at RA131 and RC132, the circuit for energizing the hold relay R140, thereby to cause the latter relay to restore shortly thereafter; and prepares, at RA 1 31 and RCI33, the previously mentioned circuit for energizing the release magnet M150. The lastmentioned circuit is completed at RAI42 and RC144 when the hold relay R140 restores and extends from ground at RA131 by way of RC133, RA142, RC144, RAI2'I, the switch springs S151 and the winding of M150 to battery. When energized over the above-traced circuit, the release magnet M150 operates to release the carriage of the Strowger mechanism included in the connector switch 40, thereby to cause the carriage to be rotated and then translated back to its normal positi-on. When the carriage of the Strowger mechanism included in the connector switch 40 is returned to its normal position, the wipers 291, 293 and 205 disengage the associated contacts in the level 200 and the switch springs S151 and S220 -are actuated into disengagement and engagement, respectively. When the switch springs S151 are actuated into disengagement, the circuit for energizing the release magnet M150 is interrupted; and when the switch springs S226 are actuated into engagement, the previously traced circuit for energizing the upper winding of the transfer relay R200 is prepared.

Also, upon restoring, the hold relay R interrupts, at RAMI and RAI45, the paths for applying ground potential to the control conductors C112 and C112', respectively. When ground potential is removed from the control conductor C112', the holding circuits for energizing the lower windings of the ring cut-off relay R210 and the switch-through relay R200 are interrupted, thereby to cause these relays to restore. Upon restoring, the relay R210 interrupts, at RA21I, RA2I3 and at RA210, RC2I'I, further points in the loop circuit extending to the called substation B; prepares, at RA2I I, RC212 and at RA210, RC218, points in the original circuit for projecting ringing current over a called subscriber line;

, prepares at RA213 and RC2I4, a point in the original circuit for returning ring-back tone current over a calling subscriber line; and interrupts, at RA2I5, a further point in the holding circuit for energizing the lower winding thereof. Finally, the relay R210 interrupts, at RA216, a

point in the previously mentioned circuit for operatingV the supervisory signal associated with the connector switch 40. The last-mentioned circuit extends from ground at RA|28 by way of RC|20, RAZl, the control conductor C222 and the winding of the supervisory signal associated with the connector switch 40 to battery. When the battery reversing relay RI20 restores, the last-mentioned circuit is completed in order to cause operation of the supervisory signal associated with the connector switch 40 in the event the ring cut-off relay R2| 0 does not restore shortly thereafter to interrupt, at RAZIS, this circuit. When the last-mentioned circuit has been completed a predetermined time interval, the supervisory signal associated with the connector switch 40 is operated, thereby to indicate to a supervisory operator that the connector switch 40 has failed to release properly.

Upon restoring, the switch-through relay R260 interrupts, at RAZ'SI, RC263 and at RAZBI, further points in the circuit for projecting ringing current over a called subscriber line; prepares, at RA204, a point in the previously traced circuit for energizing the busy relay R230;,and vinterrupts, at RA2|35, a further point in the holding circuit for energizing the lower winding thereof.

Also, when ground potential is removed fromthe control conductor CI l2, the circuit for energizing the upper winding of the switch-through relay R250 is interrupted, the line circuits ||l| and 4H' individually associated with the subscriber lines and M0 Yare released, and the iinder switch 30 is released. When the line circuits |0| and 4|? individually associated with the subscriber lines |00 and 4|0 are released, the respective subscriber lines are marked as idle to the connector switches of the links having access thereto. At this time the link is completely released for further use and the calling and called subscriber lines |00 and 4|0 are marked as idle to the connector switches of the links having access thereto.

cazzs from a substation to the P. B. X 450 A call which is to be extended from one subscriber substation to the P. B. X 450 is initiated at the calling substation by removing the receiver of the telephone instrument thereat from its hook. For example, in extending a call from the subscriber substation A to the P. B. X 450, the call is first initiated at the substation A by, removing the receiver of the telephone instrument thereat from its hook, thereby to cause the line circuit |0| individually associated with the subscriber line |00 to be operated in the previously described manner. This operation of the line circuit |0| causes the distributor 50 to assign a previously selected idle main link to the use of the calling subscriber line |00. Assuming that the distributor 50 operates to assign the previously selected idle link 20 illustrated to the use of the calling subscriber line |00, the lfinder switch of the link 20 operates', in the previously described manner, and seizes the calling subscriber line |60. When the subscriber line |00 is seized by the iinder switch 30, the line circuit |0| individually associated therewith then operates to cause the distributor to select an idle main link to the use of the next calling line inthe previously described manner. Assuming that the illustrated link 20 is assigned to the use of the calling subscriber line |00, when the nder switch 30 seizes the calling subscriber line |00, the previously traced loop circuit .between the substation A and the line, relay R is completed,thereby to cause the latter relay to operate as previously described.. Upon operating, the line relay R|30 causes the hold relay Rlli to operate, which latter relay,causesthetransfer relay R200 to operateas vpreviously explained, thereby to cause the line circuit |0| to operate further and :mark the subscriber line |00 as busy to.. the connector switches of the links having access thereto and to conditionl the connector switch 40to beresponsive -to digits dialed at the calling substation A.

The subscriber at the calling substation Ajthen dials the rst .digitofthe directory number of the main group of P. B. X trunk lines 420 extending to vthe P. B. X 450, which Vdigit inthe present instance is 0, thereby to transmit a iirst series of impulses by way of the subscriber loop circuit to the line relay R|30. The line relay R|30 follows the dialed impulses in the previously describer manner, thereby to alternately complete the previously mentioned circuit for energizing the ,hold relay R|40 and the previously traced .circuit for energizing thev transfer -relay R200 inmultiple with the vertical magnet M208. The relays -RHl and R200 being cf the slow-t0- release type remain operated during impulsing andthe vertical magnet M208 is successively energized to cause the carriage of the Strowger mechanism included in the connector switch i0 to ber-moved in the vertical direction tothe level 2%0 in the associated contact bank. When the carriage of the Strowger mechanism included in the connector switch i0 is moved away from its normal position, the switch springs S`|5| and S226 are actuated in order to prepare the previously traced circuit, for energizing the release magnet Miei); and to interrupt the previously traced circuit for energizing the upper winding of the transfer relay R200, respectively, aspreviouslynoted. Shortly following the conclusion of the rst digit, the transfer relay R200 `restores in order to prepare, at RA200 and. RC202, the previouslytraced circuit for energizing in series the lcontrol relay R260 and therotary magnet M2 0,6, as previously noted. The subscriber at the substation lA then dials the second digit ofthe directory number of the main group of P. B. X trunk lines 4|120 extending to the P. B. X450, which digit in the present instance is 2, thereby to transmit a second series of impulses by way of the subscriber loop circuit to the line relay R|30. The line relay R|30 follows thedialed impulses, thereby to alternately complete the previously mentioned circuit for energizing the hold relay Ridi) and the previously traced circuit for kenergizing inv series the control relay R240 and the rotarygmagnet M206, as .previously noted. The relaysv R|0 and .R240 remain operated during impulsing and the rotarymagnet M206 is successively energized to rotatefthe -carriage'of the Strowger mechanism includedin AtheV connector switch. 40 fto the contact set .in the vlevel 290 terminating the first P. B. X` trunk linelll inthe main group of P. B. X trunk lines 420 extending to the P. B. X 450. Upon operating'ftli'e control relay 1 R220 completes, at RAMI and RC242,- the previouslyv mentioned circuit forenergizing the control relay R250, therebyto 'cause the latter relay to. operate; and interrupts, yat RAZM, a point in a circuit, tracedV hereinafter, for energizing the control relay R220. Upon operating,A the `control relay R250 prepares,` at RA255, thefpreviously Vmentioned Acircuit forenergizing the control relay R220; interrupts, at RA256, the previously mentioned circuit for returning busy tone current over the loop circuit extending to the calling substation A; interrupts, at RA| and RC252, a point in the previously traced circuit for energizing the upper winding of the switch-through relay R260; interrupts, at RA253 and RC254, a point in the previously traced holding circuit for energizing the busy relay R230; and prepares, at RA25| and RA253, a point in the previously traced original circuit for energizing the busy relay R230.

At this time the Wipers 29|, 293, 294 and 295 carried by the carriage of the Strowger mechanism included in the connector switch 40 engage the contacts in the associated level 290 terminating therst P. B. X trunk line 42| in the main group of P. B. X trunk lines 420 extending to the P. B. X 450; the Wipers 29| and 295 engaging the contacts terminating the line conductors of the P. B. X trunk line 42|; the Wiper 293 engaging the contact terminating the control conductor of the P. B. X trunk line 42|; and the Wiper 29.4 engaging the Contact terminating the EC conductor connected to the contact terminating the control conductor of the P. B. X trunk line 42 At this time ground potential or battery potential appears upon the control conductor of the P. B. X trunk line 42| depending upon Whether this P. B. X trunk line is busy or idle, respectively. Assuming that the P. B. X trunk line 42| is busy the presence of ground potential upon the control conductor thereof completes the original circuit for energizing the busy relay R230, this circuit extending from the grounded control conductor of the P. B. X trunk line 42| by Way of the wiper 293 and associated contact, C241, RC2'I5, RA2l4, RA25I, RA253, RA264, RA27`| and the Winding of R230 to battery. When energized over the'above-traced circuit the busy relay R230 operates to interrupt, at RA23| and RC232, a point in the previously traced circuit for energizing the upper Winding of the switch-through relay R230; to prepare, at RA23| and RC233, a point in the previously traced holding circuit for energizing the Winding thereof; to prepare, at RA235, a point in the previously mentioned circuit for energizing the control relay R220; and to prepare, at RA236, a point in the previously traced circuit for returning busy tone current over the loop circuit extending to the calling substation A.

Shortly following the conclusion of the second digit dialed and the operation of the busy relay R230, the control relay R240, being of the slowto-release type, restores to complete, at RA244, the previously mentioned circuit for energizing the control relay R220, this circuit extending from the grounded control conductor of the P. B. X trunk line 42| by Way of the EC conductor illustrated, the Wiper 294 and associated contact, C248, RA2TI, RC2'l`9', RA255, RA244, RA235, MA201 and the winding of R220 to battery. When energized over the above-traced circuit, the control relay R220 completes, at RA220, a holding circuit for energizing the Winding thereof extending from the grounded control conductor C| I2 by way of RA220, MA201 and the Winding of R220 to battery. Also, upon restoring, the control relay R240 prepares, at RA24| and RC243, an alternative circuit for energizing the control relay R250, which circuit is completed at RA22| upon the operation of the control relay R220 and extends' from the grounded control conductor CI I2 by Way of RA22I, RC2 43, RA24| andthe winding of R250 to battery. Accordingly; the control relay R250 remains operated, thereby to retain interrupted, at RAZ 53 and RC254, the previously traced holding circuit for energizing the busy relay R230. Also, upon operating, the relay R220 completes, at RA22I, a circuit for energizing the rotary magnet M206, this circuit extending fromthe grounded control conductor CI |2 by Way of RA22 RA266 and the Winding of M206 to battery. When energized over the above-traced circuit, the rotary magnet M206 operates to move theV carriage of the Strowger mechanism included in the connector switch 40 an additional step in the radial direction, therebyI to cause the Wipers noted toA engage the contacts in the associated level 290 terminating the conductors of the second P. B. X trunk line 422 in the main group of P. B. X trunk lines 420 extending to the P, B. X 450. Also, upon operating, the rotary magnet M206 interrupts, at MA201, the holding circuit for energizing the control relay R220, thereby to cause the latter relay to restore. Upon restoring, the control relay R220 interrupts, at RA220, a further point in the holding circuit for energizing the Winding thereof and interrupts, at RA22I, the previously traced circuits for energizing the rotary magnet M206 and the control relay R250. The rotary magnet M206 immediately restores to prepare again, at MA201, the previously traced original circuit for energizing the control relay R220 and the control relay R250, being of the sloW-to-release type, does not immediately restore. Also, When the Wiper 293 disengages the associated contact in the level 290 terminating the control conductor of the first P. B. X trunk line 42|, the circuit for energizing the busy relay R230 is interrupted, thereby to cause this relay to restore. Furthermore, when the wiper 293 engages the associated contact in the level 290 terminating the control conductor of the second P. B. X trunk line 422, the previously traced circuit for energizing 'or short-circuiting the Winding of the busy relay R230 is completed, depending upon Whether ground or battery potentials, respectively, appear upon this control conductor, corresponding to busy or idleconditions, respectively, of the P. B. X trunk line 422. In the event the P. B. X trunk line 422 is busy, ground potential appears upon the control conductor thereof, thereby to complete the previously traced `original circuit for energizing the busy relay R230. The busy relay R230 then operates to complete, at RA235, the previously traced original circuit for energizing the control relay R220,

which latter relay then operates to complete, at

RA220, the holding circuit for energizing the Winding thereof. Further, the control relay R220 completes, vat R.A22|, the previously traced circuits for energizing the. control yrelay R250 in multiple with the rotary magnet M206. When thus energized, the rotary magnet M206 again operates to move the carriage of the Strowger mechanism included in the connector switch 40 an additional step in the radial direction, thereby tocause the wipers noted to engage the contacts in the associated'level 290 terminating the third P. B. X trunk line 4,23 in the main group of P. B. X trunk lines 420 extending to the P. B.-X 450. "It will be apparent that the operations of the relays .noted above are effective to cause the connector switch'40 to operate as atrunk hunting connector switch, which operation continues until an idle P. B.X trunk line or the last P. B. X trunk in the main group of P. B. X trunk lines 420 is seized.

In the event the third and last P. B. X trunk line 423 is busy, ground potential appearing upon the control conductor thereof completes the original circuit for energizing the busy relay R230; and the previously traced original circuit for energizing the control relay R220 is not completed due to the fact that no EC conductor is associated with the control conductor of the last P. B. X trunk line in any group of P. B. X trunk lines. Accordingly, the rotary magnet M206 is not further operated and shortly after the busy thereat that each of the P. B. X trunk lines in the main group of P. B. X trunk lines 420 extending to the P. B. X 450 is busy and that the desired connection to the P. B. X 450 may not irnmediately be had. The subscriber at the substation A then releases the operated apparatus including the line circuit and the main link 20 by replacing the receiver of the telephone instrument thereat upon its hook in the previously described manner.

Assuming that one of the P. B. X trunk lines` in the main group of P. B. X trunk lines 420 extending to the P. B. X 450 is idle when it is seized by the connector switch 40, battery potential appearing upon the control conductor thereof completes the previously traced circuit for.'-

short-circuiting the winding of the busy relay R230, thereby to prevent operation of this relay. Shortly after the idle P. B. X trunk line is seized. the control relay R250 restores to complete, at RAIV and RC`252, the previously traced circuit for energizing the upper winding of theswitchthrough relay R250, which latter relay operates to cause ringing current to be projected over the idle seized P. B. X trunk line in the previously described manner. Also, when the idle P. B. X trunk line in the main group of P. B. X trunk lines 420 is seized by the connector switch 40. the line circuit individually associated therewith in the group of line circuits 424 operates to mark the seized P. B. X trunk line as busy to the connector switches of the other main links having access thereto. The callk at the P. B. X 450is answered in the usual manner by the attendant thereat, thereby to cause the ring cut-01T relay R2|0 to be operated, as previously explained. Upon operating, the ring cut-off relay R2 I0 completes the previously traced loop circuit between the battery reversing relay RIZ!! and the seized P.V B. X trunk line extending to the P. B. X 450. The subsequent operations of the relays in the connector switch 40 incident to the operations of the ring cutol relay R2l0 and the battery reversing relay Rl20 are the same as those previ-v ously described, thereby to completev an established connection between the calling substation A and the P. B. X 450. The release of this establi'shed connection is the same as the release of the established connection previously described and is primarily under the control of the calling substation A.

Upon restoring, the control Calls from a substation to the P. B. X 460 A call which is to be extended from one Vsub- 1scriber substationv tothe P. B..X 460 is initiated at the calling substation by removing the receiver of the telephone instrumentthereat from its hook. For example, in extending a call from the subscriber substation A to the P.'B. X 460, the call is iirst initiated at the substation A by removing thereceiver of the telephone instrument there'at from its hook, thereby to cause'the line circuit l0! individually `associated with the lsubscriber line lili! to be. operated in the previously described manner. circuit i0! causes the distributor 50 to assign a previously selected idle mainlink to the use of the next calling line in the .previously described l manner.

Assuming that the illustrated link 20 is as,- signed to the use ofthe calling subscriber line m0, when. the nder switch seizes the calling subscriber line H00, the previously traced loop circuit betwen the subscriber substation A and the connector switch is completed, thereby to condition the connector switch 40 to be responsive vto digits dialed at the calling substation A and to Y cause the line circuit l0! `to operate further and 5` extending tothe P. B. X 460.

mark the subscriber line |00 as busy to the` connector switches of the links having access thereto and cause the distributor to select another idle main link to be assigned to the use of the next calling line inthe previously described manner.

The subscriber at the substation A then dials successively the two digits of the directory number of themain group of P. B. X trunk lines 430 In the present instance the directory number of this main group `of P. B. X trunk lines is 08. The operation of the connector switch i0 in response to thesuccessive digits 0 and 8 dialed thereinto is substantially the sarne as the previously described operationA thereof when the successive digits 0 and 2 are dialed thereinto. In the present instance the connector switch 40 operates as a trunk hunting connector switch to hunt for an idle P. B. X f trunk line in the main group of P. B. Xy trunk lines 430 in the same manner as it operated to hunt for an idle P. B. X trunk line in the main group of P. B. X trunk lines 420, as previouslyy described. In the event the connector switch 40 nnds an idle P. B. X trunk une in the main group of P. B. X trunk lines 430, it operates in the previously described manner in order to cause ringing current to be projected overv the idle seized P. B. X trunk line. However, in this instance the connector switch 40 does not operate immediately to cause busy tone current to be returned over the loop circuit extending to the calling substation A when the last P. B. X trunk line 433in the main group of P. B. X- trunk lines 430 is tested busy in the event that each of the P. B. X trunk lines in the main group of P. B. X trunk lines 430 is busy, as it did when the last P. B. X

Y trunk line 623 in the main group of P. B. X trunk lines 420 was tested busy.

In the present instance, when the connector switch 40 seizes the last P. B. X trunk line 433 in the main group of P. B. X trunk lines 430 extending to the P. B. X 400 and this P. B. X trunk line is tested busy, the circuit for energizing the busy relay R230 is completed as previously explained. However, the previously traced original circuit for energizing the control relay R220 is not completed due to the fact that no EC'con ductor is associated with the control conductor `of the last P;`B; Xtrunk line ll33in` the main This operation of the line group of P. B. X trunk lines 430, as previously noted. Shortly after the busy relay R230 operates, the control relay R250 restores to complete, at RA253 and RC254, the holding circuit for energizing the busy relay R230; to complete,vat RA256, the previously traced circuit for returning busy tone current over the loop circuit extending to the calling substationA; and to complete, at RA250, a circuit for energizing the upper winding of the switch-over relay R210. The last-mentioned circuit extends from the grounded conductor C223 terminated by the contact in the level 290 associated with the wiper 292 by Way of the wiper 292, C246, RA280, RA230, RA250 and the upper winding of R210 to battery. When energized over the above-traced circuit, the relay R210 operates to close the X contacts associated with the armature RA218, thereby to complete a holding circuit for energizing the lower winding thereof extending from the grounded control conductor C112 by way of RA218 and the lower winding of R210 to battery. When this holding circuit for energizing the lower winding of the relay R210 is completed, the latter relay fully operates to interrupt, at RA211, the previously traced holding circuit for energizing the busy relay R230, thereby to cause the busy relay to restore. Upon restoring, the busy relay R230 interrupts, atv RA236, a point in the previously traced circuit for returning busy tone currentv over the loop circuit extending to the calling substation A. Thus, it will be apparent that the circuit for returning busy tone current over the loop circuit extending to the calling substation A is completed and then interrupted substantially instantaneously. Accordingly, the receiver of the telephone instrument at the calling substation A is not actuated to produce busy tone.

Also, upon operating, the switch-over relay R210 interrupts, at RA211, RC212 and at RA211', RC213, points in the connections between the line conductors C134 and C135, respectively, and the line conductors C245 and C249, respectively, extending to the wipers 29| and 295, respectively; and prepares, at RA211, RC213 and at RA211', RC212', points in paths traced hereinafter for connecting the line conductors C134 and C135, respectively, to the line conductors C301 and C301, respectively, of the overflow trunk 300. Further, the relay R210 completes, at RA210, a path for applying battery potential to the line conductor C301 of the overflow trunk 300, this path extending from battery at the resistor 225, by way of the resistor 225, RA210, RC262, RA261, RA211 and RC213 to the line conductor C301. Furthermore, the relay R210 prepares, at RA214 and RC216, a circuit traced hereinafter for energizing the upper winding of the switch-through relay R260; prepares, at RA214' and RC215, a circuit traced hereinafter for energizing the lower `winding of the transfer relay R290; and completes, at RA211 and RC218, a connection between the control conductor C248 extending to the wiper 294 and the control conductor C305 of the overflow trunk 300 for a purpose more fully described subsequently. The last-mentioned connection extends from the control conductor C248 by way of RA211', RC218' and RA203 to the control conductor C305. Finally, the relay R210 completes, at RA210, a path for applying ground potential tothe start conductor C306 of the overflow trunk 300 extending to the distributor 10, this path extending from ground, at RA204, by

way of RA210" and RZS to the start conductor This application of ground potential to the start conductor C306 of the overflow trunk 300 completes a circuit for energizing the start relay R530 in series with the rotary magnet M10 in the distributor 10, this circuit extending rfrornthe grounded start conductor C306 by way of the winding of.R530,'RA531, C588, M011, MA1I and the winding of M10 to battery. Shortly after the start relay R530 is energized over the abovetraced circuit, this relay, being of the slow-tooperate type, operates to interrupt, at RA531, a point in the original circuit for energizing the winding thereof and to insert the resistor 549 in a holding circuit for energizing the relay R530 in series with the rotary magnet M10, this holding circuit extending from the grounded start conductor C306 by way of the winding of R530, the resistor 549, C588, MC11, MA1I and the winding of M10 to battery. The winding of the rotary magnet M10 is not suflciently energized over either the original circuit or the holding circuit for energizing the winding thereof in series with the winding of the start relay R530 to cause operatori thereof, due to the high series resistance ofthe winding of the start relay R530.

Also, upon operating, the start relay R530 completes, at RA532, an obvious circuit for energizing the control relay R560, thereby to cause the latter relay to operate. Upon operating, the control relay R560 prepares, at RA56I, a circuit traced hereinafter for energizing the lower winding of the transfer control relay R580 in series with the lower winding of the stepping relay R515; prepares, at RA562, a circuit traced hereinafter for energizing the upper winding of the stepping relay R515;' prepares, at RA564, a circuit traced hereinafter for energizing the winding of the switch-through control relay R510 in series with the lower winding of the stepping relay R515; prepares, at RA565, a circuit traced hereinafter for energizing the upper winding of the transfer control relay R580; prepares, at RA561, a path traced hereinafter for applying ground potential to thetrunks busy conductor C396; and prepares, at RA568, a first' circuit traced hereinafter for energizing the rotary magnet M10. Furthermore, the control relay R560 completes, at RA566, a circuit for energizing the lower winding of the control relay R550, this circuit extending from ground at RA566 by way of RA542, RC544 and the lower winding of R550 to battery. When energized over the above-traced circuit, the control relay R550 'operates 'to complete, at RA551, a circuit for energizing the timing relay R555, this circuit extending from ground at RA566 by way of RA541, RA551, and the winding of R555 to battery. Further, the control relay R550 prepares, at RA552, a circuit, traced hereinafter, for energizing the upper winding thereof; interrupts, at RA553, a point in the previously mentioned path for applying ground potential to the trunks busy conductor C396; and interrupts, at RA554, a pointy in the previously mentioned circuit for energizing the rotary magnet M10. When energized over the abovetraced circuit the timing relay R555 operates to set the weighted armature RA556 thereof into operation in order to cause this vibrating armature to engage alternately the associated contacts RC551 and RC558 arranged on opposite sides thereof. The alternate engagements of the weighted armature RA556 with the contacts RC551 and RC558 intermittently come plete circuits for energizing ythe upper winding of the control relay R550 and theupper winding of the vtrunks b usy relay R550. The first-mentioned circuit extends fromground vat RA550 by way of RA552, RA555, and RC551 or RC558 and the upper winding of R550 to battery; and the last-mentioned circuit extends from ground at RA555 by way of RA552, RA556 and RC551 or RC558, RC555, RA543 and the upper winding of R555 to battery. `Due to the facts that the abovetraced circuit for energizing the upper winding of the trunks busy relay R550 is only intermittently completed and that the latter relay is of the slowto-operate type, the lower winding thereof being short-circuited under the control of vthe armature RASS, the trunks busy relay R550 does not immediately operate.

Finally, the control relay R550 completes, at RA553, a circuit for energizing the rotary magnet Ml, this circuit extending from ground at R516 by way of RA553, C553, the wiper 505 and Contact in the associated contact bank 505 terminating the control conductor C393, C393, RA385, RC381 and the winding of M5 I to battery. When energized over thefabove-traced circuit, the rotary magnet MSI operates to condition the wipers 322, 32B, 332, 335-, 352 and 355 of the rotary nder switchv 6I to be operated-one step in a counterclockwise direction and completes, at MAS! and MCBI, the previously mentioned circuit for energizing the upper winding of the stepping relay R515, this circuit extending from ground `at MCfI-I by way of MAM, the control conductor C394, the wiper 542 and contact in the associated contact bank terminatinglCSM, C589,

RA55?. and. the upper winding of R515 to battery.V

When energized over the above-traced circuit, the stepping relay R515 operates to interrupt, at RA516, the previously traced circuit for energizing the rotary'magnet MGI thereby to cause the rotary magnet MBI to restore. Upon restoring, the rotary magnet MBI causesthe wipers noted of the rotary nder switch 6| to be stepped one step in the counterclockwise direction and into engagement with the respective contacts in the associated contact banks terminating the conductors of an overflow trunk'extending thereto from the connector switch of one of the main links. Also, upon restoring, the rotary magnet M5! interrupts, at MASI and MC5I, the previously traced circuit for energizing the stepping relay R515, which latter relay restores to cornplete, at RA515, the previously traced circuit for energizing the rotary magnet MSI. The rotary magnet Mtl again operates to condition the wipers of the rotary nder switch 0I to be rotated an additional step in the counterclockwise direction and to complete the previously traced circuit for energizing-the upper winding of the stepping relay R515. The rotary magnet 5I and the stepping relay R515 continue to interact in the above-described manner until the wipers noted of the rotary finder switch 0I engage the respective contacts in the associated Contact banks terminating the conductors ofthe overflow trunk 300 extending to the connector switch 40 of the main link 20, at which time the previously mentioned circuit Vfor energizing in series the lower windings of the transfer control relay R580 and the stepping relay R515 is completed. `The last-mentioned circuit extends from batterypotential appearing upon the line conductor C35I of the overflow trunk 330 by way of the Contact in the contact bank 32| terminating C35 I, the associated wiper 322, RA31 I RC312,

" with the respective contacts in the associated contact banks terminating the conductors of the RA38I, the control conductor C305, the contact in the contact` bank 5I5 terminating C395, the associated wiper 5l5,VC584, RA56I`, the lower Winding of R580 and the lower winding of R515 to battery. This circuit constitutes a holding.

circuit for energizing the lower winding of the stepping relay R515, thereby to prevent the stepping relay R515 from restoring when the rotary magnet MEI is restored to drive the wipers notedv of the rotary nder switch 0I into engagement overflow trunk 300.

When energized over the above-traced circuit,

i the transfer control relay R580 operates to complete, at RA532,.the previouslyrnentioned holding circuit for energizing Vthe upper winding thereof, this circuit extending from ground at RA555 by way of RA582 and thegupper winding of R585 to battery. vAlso, upon operating, the

" relay R580 completes, at RA58l, a circuit for energizing the transfer relay R380, this circuit extending from ground at RAEBIwby way of C585, the wiper 526, the contact in the associated contact bank 525 terminating the control conductor C353, C398 and the winding of R380 to battery.

When energized over the above-traced circuit, the transfer relay R380 operates to interrupt, at

RA38I, a point in the'previously traced circuit for energizing the lower winding of the transfer control relay R580 in series with the lower wind-` ing of `the stepping relayR515; to prepare, at RA332, a further point in the previously mentioned circuit for energizing -thewinding of thev switch-through control relay R510 in series with the lower winding of the stepping relay R515; to

complete, at RA383, an obvious connection between the wipers362 and 355 included inthe rotaryhunting switch 52; and to complete, at RA335, a connection between the trunks busy conductor C303 in the overflow trunk 305 and hereinafter` for energizing the rotary magnet When the previously traced circuit for energizing the lower windings of the relays R535 and R515 is interrupted, the relay'R515 restores to `conlplete. at RA516, the previously mentioned tact bank 505 terminating the control conductor circuit for energizing the winding of therotary magnet M52. The last-mentioned circuit extends from ground at RA515 by way of RA563, C583, the wiper 535, the'contact in the associated con- 0393,C353, RA305, RCIiEi and the winding of M52 tol battery. When energized over the abovev traced circuit, the rotary magnet M02 operates to condition the wipers 352, 355, 302 and 355 of the rotary hunting switch 52 to be rotated one step in a clockwise direction. Also, 4upon operating. the rotary magnet M52 completes, at MA52 and MCM, an alternative circuit vfor energizing the upper winding of the stepping relay R515,

` this alternative circuit extending from ground at MC52 by way of M Aii, the control conductor C3345, the contact in the contact bank 5I I terminating C395, the associated wiper 5I2, C583,

; RA552 and the upper Winding of R515 to battery.`

When energized over the above-traced circuit, the stepping relay R515 operates to interrupt, at RA516, the previously traced circuit for energizing the rotary magnet M62, thereby to cause the rotary magnet M62 to restore. Upon restoring, the rotary magnet M62 drives the wipers noted of the rotary hunting switch 62 one step in the clockwise direction; and interrupts, at MA62 and MC62, the previously traced alternative circuit for energizing the upper winding of the 'stepping relay R515, thereby to cause the latter relay to restore and complete, at RA516, the previously traced circuit for energizing the rotary magnet M62 in order to cause the rotary magnet M62 to operate and condition the wipers noted of the rotary hunting switch 62 to he rotated an additional step in the clockwise direction and to complete the previously traced alternative circuit for energizing the upper winding of the stepping relay R515, thereby to cause operation thereof. The stepping relay R515 and the rotary magnet M62 interact, in the manner described above, in order to cause the wipers noted of the rotary hunting switch 62 to be moved step by step in a clockwise direction until an idle P. B. X trunk line extending to the P. B. X 460 is found or until the elapse of a predetermined time interval. More particularly, when an idle P. B. X trunk line extending to the P. B. X 460 is found, the previously mentioned circuit for energizing in series the switch-through control relay R510 and the lower winding of the stepping relay R515 is completed in a manner more fully described subsequently. It is pointed out that as the wipers noted of the rotary hunting switch 62 are rotated, the respective contacts in the associated contact banks terminating the auxiliary group of trunk lines 440 are successively engaged and subsequently the respective contacts in the associated Contact banks terminating the main group of P. B. X trunk lines 430 are successively engaged. Thus, it will be apparent that the hunting switch 62 first tests successively the idle or busy condition of the P. B. X trunk lines 44|, 442 and 443 of the auxiliary group of trunk lines 440 extending to the P. B. X 460 and subsequently tests successively the P. B. X trunk lines 43|, 432 and 433 in the main group of P. B. X trunk lines 430 extending to the P. B. X 460. Furthermore, it is pointed out that the wipers noted of the hunting switch 62 are rotated at least once during the operation ofthe hunting switch 62 before the elapse of the predetermined time interval mentioned. Accordingly, both ends of the wipers noted of the hunting switch 62 are rotated across the associated contact banks in the event no idle P. B. X trunk line extending to the P. B. X 466 |s found, whereby both the auxiliary and main groups of P. B. X trunk lines extending to the P. B. X 460 are tested at least twice before the elapseof the predetermined time interval mentioned. Thus, the hunting switch 62 is cyclically operative; and, preferably, the rate of operation thereof is so correlated with respect to the predetermined time interval mentioned that this switch is operative through a predetermined number of cycles of operation during the predetermined time interval mentioned.

During the operation of the hunting switch 62 the wiper 362 successively engages in the associated contact bank 36| the contacts terminating the control conductors of the P. B. X trunk lines in the auxiliary and main groups of P. B. X trunk lines 440 and 430, respectively, extending to the P. B. X 460. When the wiper 362 engages in the associated contact bank 36| a contact terminating the control conductor of Ya busy P. B. X trunk line extending to the P. B. X 460, ground potential appearing upon the control conductor completes a path for short-circuiting in series the switch-through control relay R510 and the lower winding of the stepping relay R515. This circuit extends from the grounded control conductor of the P. B. X trunk line by way of the contact in the contact bank 36| terminating this control conductor, the associated wiper 362, RA383, the wiper 366 and the multipled contacts in the associated contact bank 365, the control conductor C3l5, the wiper 294 and associated contact in the level 290 terminating C3|5, C248, RA211', RC218', RA203, the control conductor C305 of the overilow trunk 300, the wiper 336 and associated contact in the contact bank 335 terminating C305, RA382, C391, RA564, the winding of R510 and the lower winding of R515 to ground.

On the other hand, when the wiper 362 engages in the associated contact bank 36| a contact terminating the control conductor of an idle P. B. X trunk line extending to the P. B. X 460, battery potential appearing upon the control conductor completes a circuit for energizing in series the switch-through control relay R510 and the lower winding of the stepping relay R515, this circuit being substantially identical to the previously traced rcircuit for short-circuiting in series the switch-through control relay R510 and the lower winding of the stepping relay R515. It is noted that the previously mentioned circuit for energizing the winding of the switch-through control relay R510 in series with the lower winding of the stepping relay R515, which circuit when completed causes further operation of the rotary magnet M62 to be arrested and the overilow link 60 to be switched through, includes the control conductor C3|5 terminated at the multipled contacts in the contact bank 365 of the rotary hunting switch 62; `and that the multipled contacts mentioned are engaged by the associated wiper 366 only during the portion of the cycleof operation of the rotary hunting switch 62 while the wiper 362 is engaging in the associated contact bank 36| the contacts terminating the control conductors of the P. B. X trunk lines in the auxiliary and main groups of P. B. X trunk lines 440 and 430, respectively, extending to the P. B. X 460. Accordingly, at this time, the hunting switch 62 effectively tests only the P. B. X trunk lines extending to the P. B. X 460; and the previously mentioned energizing circuit can be completed only in the event that an idle P. B. X trunk line extending to the P. B. X 460 is found. This arrangement positively prevents, at this time, the overiiow link 60 from being switched through due to the presence in the contact banks of the rotary hunting switch 62 thereof of an idle P. B. X trunk line extending to another P. B. X, such, for example, as the P. B. X 410.

When the above-mentioned circuit for energizing in series the winding of the switch-through control relay R510 and the lower winding of the stepping relay R515is completed, the stepping relay R515 is retained in its operated position and does not restore incident to the interruption of the circuit for energizing the upper winding thereof, when the rotary magnet M62 is restored in order to cause the wipers noted of the rotary hunting switch 62 to be driven an additional step inthe clockwise directionv and into engagement with the contacts in the associated' contact banks terminating the conductors of the'idle P. B. X trunk line in either the auxiliary orr main groups of P. B. X trunk lines 440 or 430, respectively, extending to the P. B. X 460. Accordingly, the previously traced circuit for energizing the rotary magnet M62 is not again completed and further operation of the rotary hunting 'switch 52 is positively arrested.

Also, when the above-mentioned circuit for energizing in series the switch-through control relay R510 and the lower winding of the stepping magnet R515 is completed, the relay R510 operates to complete, at RA51I, the previously mentioned circuit for energizing the upper winding of the switch-through relay R310. This circuit extends from ground at RA51| by way of C585, the wiper 522 and contact in the associated contact bank 52| terminating the control conductor C392, C392 and the upper winding of R310 to battery. When energized over the abovetraced circuit. the switch-through relay R310 operates to complete, at RA316, a holding circuit for energizing the lower winding thereof in series with the lower winding of 'the transfer relay R200 in the connector switch 40 of the main link 20. This circuit extends from ground at the lowerwinding of R310 by way of the lower winding of R310, RA316, the wiper 346 and contact in the associated contact bank 345 terminating the control conductor C304 of the overflow trunk 3.00, C304, RC215, RA214, and the lower winding of R200 to battery. Also, upon operating, the switch-through relay R310 completes, at RA315, a circuit for energizing the upper winding of the switch-through relay R200 in the connestor switch 40 of the main link 20, this circuit extending from the grounded control conductor Cl i2, by way of RA23|, RC232, the upper winding of R200, RC252, RA25I, RA214, RC210, the control conductor C302 of the overflow trunk 300, the Contactin the contact bank 33| terminating C302 and the associated wiper 332, RA315, the wiper 302 and contact in the associated contact bank 35| terminating the control conductor oi the idle P. B. X trunk line seized, battery potential appearing upon this control conductor. Also, when the idle P. B. X trunk line in the main or auxiliary groups of P. B. X trunk lines 330 or 450, respectively, is seized by the hunting switch 02 the line circuit individually associated therewith in the group of line circuits 434 operates to mark the seized main P. B. X trunk line as busy to the connector switches oi the main links and the hunting switches of the other overow links or to mark the seized auxiliary P. B. X

trunk line as busy tothe hunting switches of the other overow links. Further, the switchthrough relay R310 completes, at RA31I, RC313 and at RA314, connections between the line conductors of the seized P. B. X trunk line and the line conductors C| and C301 of the overflow trunk 300. The connection between one of the line conductors of the-seized P. B. X trunk line and the line conductor C30! of the overflow trunk 300 extends from the .one line conductor of the P. B. X trunk line by way of the contact in the contact bank terminating the one line conductor and the associated wiper 352, RC313, RA3'il, the wiper 322 and the contact in the associated contact bank 32| terminating C30| to the line conductor C30! ;,and the connection between the other line conductor of the seized P. B. X trunk line and the other line conductor C301 of the overflow trunk 300 extends from the f otherP. B.. X trunk line by wayof the contact in the contact bank 355 terminating the other line conductor and the associated wiper 350, RA314, the Wiper 326'and the contact in the associated contactbank 325 terminating C301 to the line conductor C301.

When the upper Winding of the switch-through relay R200 in the connector Aswitch 40 is energized over the above-traced circuit, thi-s relay partially operates to close the X contacts associated with the armature RA205, thereby to complete the previously traced holding circuit for energizing the lowerwinding thereof. When the holding circuit for energizing the lower winding of therelay R200 is completed, this relay fully operates to complete, at RA260, the previously traced circuit for returning ring-back tone current over the loop circuit extending to the calling substation A; to interrupt at RA20| and RC202, a further point in the previously traced n circuit for energizing in series the lower windings of the transfer controlrelay R580 and the stepping relay R515 inthe distributor 10. Further, `the switch-through relay R260 completes, at RA20I, RC263 and at RA261, a circuit for projecting ringing current over the seized P. B. X trunk lineextending to the P. B. X 400, this circuit extending from ground at RC2|8 by way of RA2|0, --RA201, RA21I' and RC212 to the line conductor C301 of the overflow trunk 300; and from the ungrounded terminal of the grounded ringing current transformer winding by way of the ringing current conductor C221, the upper winding of the ring cut-01T relay R2|0, RC2|2, RA2| RC263, RAZGI, rRA21| and RC213 to the line conductor C30| of the overflow trunk 300, the line conductors C301 and C30| of the overow trunk 300 being connected to the line conductors of the seized P. B. X trunk line by way of the operated overiiow link over the previ-l ously traced connections. The ringing current projected by way of the above-traced circuit over the P. B. X trunk line extending to the P. B. X 460 causes the signal thereat to be operated, thereby to indicate to the attendant at the P. B. X 400 that an incoming ycall isextended thereto. The call is answered at the P. B. X 460 in the usual manner to complete a circuit substantially identical to the above-traced circuit for projecting ringing current over the seized P. B.X trunk line for' energizing the upper winding of the ring cut-cifrelay R2|0. When the upper winding of the ring cut-oil relay R2 l0 is energized, this relay partially operates to close the X contacts associated with the armature RA2|5, thereby to complete the previously traced holding circuit for* energizing the lower winding thereof. When the'holding circuit for energizing the lower winding of the ring cut-off relay R2|0 is thus completed, this relay fully operates to interrupt, at RA2| I, RC2|2 and at RA2|0, RCZIB, the previously traced circuit for projecting ringing current over the seized P. B. Xtrunk line; to interrupt, aty RA2|3 and RC.2|4, the previously traced circuit .for returning. ring-back tone current over the loop circuit extending to the calling substation A; and to complete, at RA2| RA2| 3 and at RA2|0, RC2I1, a loop circuit extending between the battery reversing relay R|20 and the P. B. X 400, this loop circuit extending from ground at the lower winding of R|20 by yway of the lower winding of R|20, C|35,l RC2|1RA2|.0, RA201, RA21l' `andRCZ'IZ to the line conductor C301 of the overiiowtrunk 300; and from battery 

